Particulate metal oxide

ABSTRACT

A particulate metal oxide having a coating layer comprising phosphate wherein the mean length of the primary particles is in the range from 50 to 90 nm, and the mean width of the primary particles is in the range from 5 to 20 nm. The metal oxide is particularly suitable for forming aqueous dispersions. The metal oxide and aqueous dispersions thereof can be used in a sunscreen product that exhibits both effective UV protection and improved transparency.

FIELD OF INVENTION

The present invention relates to a particulate coated metal oxide, ametal oxide dispersion and in particular to the use thereof in asunscreen product.

BACKGROUND

Metal oxides such as titanium dioxide, zinc oxide and iron oxides havebeen employed as attenuators of ultraviolet light in applications suchas sunscreens, plastics films and resins. Due to the increased awarenessof the link between ultraviolet light and skin cancer, there has been anincreasing requirement for ultraviolet light protection in everydayskincare and cosmetics products. Unfortunately, existing commerciallyavailable metal oxide products, such as titanium dioxide, are notsufficiently transparent and can have an unacceptable whitening effectwhen used on the skin. There is a need for a metal oxide which exhibitsimproved transparency, reduced whitening, and provides broad spectrumultraviolet light protection. There are particular problems involved inachieving the aforementioned properties in aqueous media.

Review of the Prior Art

GB-2205088-A discloses particulate acicular titanium dioxide having acoating layer of aluminium oxide and silicon oxide.

GB-2226018-A is directed to an aqueous dispersion of particulateacicular titanium dioxide containing an acrylic dispersing agent.

SUMMARY OF THE INVENTION

We have now surprisingly discovered an improved metal oxide, whichovercomes or significantly reduces at least one of the aforementionedproblems.

Accordingly, the present invention provides a particulate metal oxidehaving a coating layer comprising phosphate wherein the mean length ofthe primary particles is in the range from 50 to 90 nm, and the meanwidth of the primary particles is in the range from 5 to 20 nm.

The present invention also provides a dispersion comprising particles ofmetal oxide having a coating layer comprising phosphate in a dispersingmedium wherein the mean length of the primary particles is in the rangefrom 50 to 90 nm, and the mean width of the primary particles is in therange from 5 to 20 nm.

The invention further provides a particulate metal oxide having acoating layer comprising phosphate, and having an extinction coefficientat 524 nm (E₅₂₄) in the range from 0.2 to 1.5 I/g/cm, an extinctioncoefficient at 450 nm (E₄₅₀) in the range from 0.1 to 2.0 Vg/cm, anextinction coefficient at 360 nm (E₃₆₀) in the range from 5 to 10I/g/cm, an extinction coefficient at 308 nm (E₃₀₈) in the range from 35to 65 I/g/cm, a maximum extinction coefficient E(max) in the range from50 to 80 I/g/cm, and a λ(max) in the range from 260 to 290 nm.

The invention further provides a sunscreen product comprising a metaloxide or dispersion as defined herein.

The invention still further provides the use of a metal oxide ordispersion as defined herein in the manufacture of a sunscreen havingreduced whiteness.

Preferably the metal oxide used in the present invention comprises anoxide of titanium, zinc or iron, and most preferably the metal oxide istitanium dioxide.

The preferred titanium dioxide particles comprise anatase and/or rutilecrystal form. The titanium dioxide particles preferably comprise a majorportion of rutile, more preferably greater than 60% by weight,particularly greater than 70%, and especially greater than 80% by weightof rutile, based on the total weight of titanium dioxide. The titaniumdioxide particles preferably comprise in the range from 0.01 to 5%, morepreferably 0.1 to 2%, and particularly 0.2 to 0.5% by weight of anatase,based on the total weight of titanium dioxide. In addition, the titaniumdioxide particles preferably comprise less than 40%, more preferablyless than 30%, and particularly less than 25% by weight of amorphoustitanium dioxide, based on the total weight of titanium dioxide. Thebasic particles may be prepared by standard procedures, such as usingthe chloride process, or by the sulphate process, or by hydrolysis of anappropriate titanium compound such as titanium oxydichloride or anorganic or inorganic titanate, or by oxidation of an oxidisable titaniumcompound, e.g. in the vapour state. The titanium dioxide particles arepreferably prepared by the hydrolysis of a titanium compound,particularly of titanium oxydichloride.

The particles of metal oxide according to the present invention have acoating layer, preferably inorganic, comprising phosphate. The preferredamount of phosphate in the coating layer is in the range from 0.1 to12%, more preferably 0.5 to 6%, particularly 1 to 3%, and especially 1.5to 2.5% by weight of phosphorous, based on the weight of metal oxidecore particles.

The coating layer may be formed by adding a, preferably water soluble,phosphate or salt of phosphoric acid to a dispersion, normally aqueous,to uncoated metal oxide core particles. Suitable water solublephosphates include metal, preferably alkali metal, or ammoniumphosphates such mono-, di- or tri-sodium phosphate, mono-, di- ortri-potassium phosphate or alternatively a polymeric phosphate, such asa polymeric alkali metal phosphate, for example tri-sodium polyphosphateor sodium hexametaphosphate. Polymeric phosphates are preferred,particularly sodium hexametaphosphate.

The coating layer is preferably formed by precipitating the phosphatewith a suitable cation, preferably metal, on to the surface of the metaloxide particles. Suitable metal cations include aluminium, zirconium andcerium, and preferably aluminium. The precipitation of the phosphate,preferably metal phosphate, and particularly aluminium phosphate, can beachieved by adjusting the pH of the dispersion by the addition of acidor alkali, as appropriate. The amount of the preferred metal,particularly aluminium, in the coating layer is preferably in the rangefrom 0.2 to 20%, more preferably 1.5 to 10%, particularly 3 to 7%, andespecially 4 to 5% by weight, based on the weight of metal oxide coreparticles.

The amount of phosphorous in the coating layer is preferably in therange from 2 to 35%, more preferably 5 to 25%, particularly 8 to 20%,and especially 10 to 14% by weight based on the weight of the coatinglayer. The amount of metal, preferably aluminium, in the coating layeris preferably in the range from 10 to 55%, more preferably 20 to 50%,particularly 25 to 45%, and especially 30 to 40% by weight based on theweight of the coating layer.

The preferred metal compound, more preferably water soluble,particularly in the form of a salt such as a sulphate or oxide,preferably sulphate, can be added together with the phosphate to thedispersion of metal oxide particles. In a preferred embodiment of theinvention, the coating layer is formed by adding sodiumhexametaphosphate and aluminium sulphate to an aqueous slurry ordispersion of metal oxide particles, and the pH adjusted in order toachieve precipitation of aluminium phosphate.

In a preferred embodiment of the invention, the coating layer comprisesaluminium oxide and/or aluminium hydroxide (hereinafter both referred toas alumina), in addition to the aforementioned phosphate, preferablyaluminium phosphate. The amount of alumina present in the coating layeris preferably in the range from 1 to 20%, more preferably 3 to 14%,particularly 6 to 11%, and especially 7 to 9% by weight, based on theweight of metal oxide core particles. Alumina can be formed as acomponent in the coating layer by adding aluminium sulphate and/or ametal aluminate, preferably water soluble, to the dispersion of metaloxide particles. Sodium aluminate is a particularly preferred metalaluminate. Precipitation of alumina on the surface of the metal oxideparticles can also be achieved by suitable control of pH.

The ratio by weight of the preferred aluminium phosphate to aluminapresent in the coating layer is suitably in the range from 0.1 to 1.8:1,preferably 0.2 to 1.2:1, more preferably 0.3 to 0.8:1, particularly 0.4to 0.65:1, and especially 0.5 to 0.6:1.

Thus, in a particularly preferred embodiment of the invention, thecoating layer is formed by adding sodium hexametaphosphate, aluminiumsulphate and sodium aluminate to a dispersion of metal oxide particles,and the pH adjusted to in order to achieve precipitation of bothaluminium phosphate and alumina.

The individual or primary metal oxide particles are preferably acicularin shape and have a long axis (maximum dimension or length) and shortaxis (minimum dimension or width). The third axis of the particles (ordepth) is preferably approximately the same dimensions as the width. Thesize of the primary particles can be suitably measured using electronmicroscopy. The size of a particle can be determined by measuring thelength and width of a filler particle selected from a photographic imageobtained by using a transmission electron microscope. Mean values can bedetermined from the measurements of at least 300 particles, as describedherein.

The mean length by number of the primary metal oxide particles is in therange from 50 to 90 nm, preferably 55 to 77 nm, more preferably 55 to 73nm, particularly 60 to 70 nm, and especially 60 to 65 nm. The mean widthby number of the particles is in the range from 5 to 20 nm, preferably 8to 19 nm, more preferably 10 to 18 nm, particularly 12 to 17 nm, andespecially 14 to 16 nm.

The size distribution of the primary metal oxide particles can also havea significant effect on the final properties of, for example, asunscreen product comprising the metal oxide. In a preferred embodimentof the invention suitably at least 40%, preferably at least 50%, morepreferably at least 60%, particularly at least 70%, and especially atleast 80% by number of particles have a length within the abovepreferred ranges given for the mean length. In addition, suitably atleast 40%, preferably at least 50%, more preferably at least 60%,particularly at least 70%, and especially at least 80% by number ofparticles have a width within the above preferred ranges given for themean width.

The primary metal oxide particles suitably have a mean aspect ratiod₁:d₂ (where d₁ and d₂, respectively, are the length and width of theparticle) in the range from 2.0 to 8.0:1, preferably 3.0 to 6.5:1, morepreferably 4.0 to 6.0:1, particularly 4.5 to 5.5:1, and especially 4.5to 5.0:1.

The primary metal oxide particles suitably have a median volume particlediameter (equivalent spherical diameter corresponding to 50% of thevolume of all the particles, read on the cumulative distribution curverelating volume % to the diameter of the particles—often referred to asthe “D(v,0.5)” value), measured as herein described, in the range from20 to 35 nm, preferably 23 to 33 nm, more preferably 25 to 31 nm,particularly 25 to 28, and especially 25 to 26 nm.

When formed into a dispersion, the particulate metal oxide according tothe present invention suitably has a median volume particle diameter(equivalent spherical diameter corresponding to 50% of the volume of allthe particles, read on the cumulative distribution curve relating volume% to the diameter of the particles—often referred to as the “D(v,0.5)”value)) (hereinafter referred to as dispersion particle size), measuredas herein described, of less than 45 nm, preferably less than 40 nm,more preferably in the range from 17 to 35 nm, particularly 20 to 30 nm,and especially 22 to 25 nm.

The size distribution of the metal oxide particles in dispersion canalso be an important parameter in obtaining, for example, a sunscreenproduct having the required properties. The metal oxide particles indispersion suitably have no more than 16% by volume of particles havinga volume diameter of less than 14 nm, preferably less than 16 nm, morepreferably less than 18 nm, particularly less than 20 nm, and especiallyless than 22 nm. In addition, the metal oxide particles in dispersionsuitably have more than 84% by volume of particles having a volumediameter of less than 45 nm, preferably less than 38 nm, more preferablyless than 34 nm, particularly less than 30 nm, and especially less than26 nm.

It is preferred that in dispersion none of the metal oxide particlesshould have an actual particle size exceeding 150 nm. Particlesexceeding such a size may be removed by milling processes which areknown in the art. However, milling operations are not always totallysuccessful in eliminating all particles greater than a chosen size. Inpractice, therefore, the size of 95%, preferably 99% by volume of theparticles should not exceed 150 nm.

The dispersion particle size of the matal oxide particles describesherein may be measured by electron microscope, coulter counter,sedimentation analysis and static or dynamic light scattering.Techniques based on sedimentation analysis are preferred. The medianparticle size may be determined by plotting a cumulative distributioncurve representing the percentage of particle volume below chosenparticle sizes and measuring the 50th percentile. The median particlevolume diameter of the metal oxide particles in dispersion is suitablymeasured using a Brookhaven particle sizer, as described herein.

In a particularly preferred embodiment of the invention, the metal oxideparticles have a BET specific surface area, measured as describedherein, of greater than 40, more preferably in the range from 50 to 140,particularly 60 to 120, and especially 65 to 100 m²/g.

The metal oxide particles exhibit improved transparency preferablyhaving an extinction coefficient at 524 nm (E₅₂₄), measured as hereindescribed, of less than 2.0, more preferably in the range from 0.2 to1.5, particularly 0.4 to 1.0, and especially 0.5 to 0.8 l/g/cm. Inaddition, the metal oxide particles preferably have an extinctioncoefficient at 450 nm (E₄₅₀), measured as herein described, of less than3.0, more preferably in the range from 0.1 to 2.0, particularly 0.5 to1.5, and especially 0.7 to 1.0 I/g/cm.

The metal oxide particles exhibit effective UV absorption, suitablyhaving an extinction coefficient at 360 nm (E₃₆₀), measured as hereindescribed, of greater than 3, preferably greater than 4, more preferablyin the range from 5 to 10, particularly 5.5 to 8, and especially 6 to7.5 I/g/cm. The metal oxide particles also suitably have an extinctioncoefficient at 308 nm (E₃₀₈), measured as herein described, of greaterthan 30, preferably in the range from 35 to 65, more preferably 40 to60, particularly 45 to 55, and especially 46 to 50 I/g/cm.

The metal oxide particles suitably have a maximum extinction coefficientE(max), measured as herein described, of greater than 45, preferably inthe range from 50 to 80, more preferably from 55 to 75, particularly 60to 70, and especially 65 to 70 l/g/cm. The metal oxide particlessuitably have a λ(max), measured as herein described, in the range from260 to 290, preferably 265 to 285, more preferably 268 to 282,particularly 270 to 280 nm, and especially 275 to 280 nm.

The metal oxide particles suitably exhibit reduced whiteness, preferablyhaving a change in whiteness ΔL of a sunscreen product containing theparticles, measured as herein described, of less than 3, more preferablyin the range from 0.5 to 2.5, and particularly 1.0 to 2.0. In addition,a sunscreen product containing the particles preferably has a whitenessindex, measured as herein described, of less than 100%, more preferablyin the range from 10 to 80%, particularly 20 to 60%, and especially 30to 50%.

The metal oxide particles suitably have reduced photogreying, preferablyhaving a photogreying index, measured as herein described, of less than15, more preferably in the range from 1 to 10, particularly 2 to 7, andespecially 3 to 5.

A composition, preferably a sunscreen product, containing the metaloxide particles defined herein suitably has a Sun Protection Factor(SPF), measured as herein described, of greater than 10, preferablygreater than 15, more preferably greater than 20, particularly greaterthan 25, and especially greater than 30 and up to 40.

The metal oxide particles according to the present invention provide asurprising combination of both improved photostability anddispersibility, particularly when dispersed in aqueous media.

The particulate metal oxide according to the present invention may be inthe form of a free-flowing powder. A powder having the preferredparticle sizes for the metal oxide particles, as described herein, maybe produced by milling processes known in the art. The final millingstage of the metal oxide is suitably carried out in dry, gas-bomeconditions to reduce aggregation. A fluid energy mill can be used inwhich the aggregated metal oxide powder is continuously injected intohighly turbulent conditions in a confined chamber where multiple, highenergy collisions occur with the walls of the chamber and/or between theaggregates. The milled powder is then carried into a cyclone and/or bagfilter for recovery. The fluid used in the energy mill may be any gas,cold or heated, or superheated dry steam.

The particulate metal oxide according to the present invention may beformed into a slurry, or preferably a liquid dispersion, in any suitableaqueous or organic liquid medium. By dispersion is meant a truedispersion, ie where the solid particles are stable to aggregation. Theparticles in the dispersion are relatively uniformly dispersed andresistant to settling out on standing, but if some settling out doesoccur, the particles can be easily redispersed by simple agitation.

Cosmetically acceptable materials are preferred as the liquid medium. Auseful organic medium is a liquid oil such as vegetable oils, e.g. fattyacid glycerides, fatty acid esters and fatty alcohols. A preferredorganic medium is a siloxane fluid, especially a cyclic oligomericdialkylsiloxane, such as the cyclic pentamer of dimethylsiloxane knownas cyclomethicone. Alternative fluids include dimethylsiloxane linearoligomers or polymers having a suitable fluidity andphenyltris(trimethylsiloxy)silane (also known as phenyltrimethicone).The particulate metal oxide is particularly suitable for use in aqueousmedia, and may be formed into an aqueous slurry, or preferably anaqueous dispersion. A surprising feature of the present invention isthat aqueous dispersions can be produced which contain at least 30%,preferably at least 35%, more preferably at least 40%, particularly atleast 45%, especially at least 50%, and generally up to 55% or even 60%by weight of the total weight of the dispersion, of metal oxideparticles as described herein.

The metal oxide dispersions may also contain a dispersing agent in orderto improve the properties thereof. The dispersing agent is suitablypresent in the range from 1 to 50%, preferably 2-to 30%, more preferably3 to 20%, particularly 4 to 15%, and especially 5 to 10% by weight basedon the total weight of metal oxide particles.

Suitable dispersing agents for use in an organic medium includesubstituted carboxylic acids, soap bases and polyhydroxy acids.Typically the dispersing agent can be one having a formula X.CO.AR inwhich A is a divalent bridging group, R is a primary secondary ortertiary amino group or a salt thereof with an acid or a quaternaryammonium salt group and X is the residue of a polyester chain whichtogether with the —CO— group is derived from a hydroxy carboxylic acidof the formula HO—R′—COOH. As examples of typical dispersing agents arethose based on ricinoleic acid, hydroxystearic acid, hydrogenated castoroil fatty acid which contains in addition to 12-hydroxystearic acidsmall amounts of stearic acid and palmitic acid. Dispersing agents basedon one or more polyesters or salts of a hydroxycarboxylic acid and acarboxylic acid free of hydroxy groups can also be used. Compounds ofvarious molecular weights can be used. Other suitable dispersing agentsare those monoesters of fatty acid alkanolamides and carboxylic acidsand their salts. Alkanolamides are based on ethanolamine, propanolamineor aminoethyl ethanolamine for example. Alternative dispersing agentsare those based on polymers or copolymers of acrylic or methacrylicacids, e.g. block copolymers of such monomers. Other dispersing agentsof similar general form are those having epoxy groups in the constituentradicals such as those based on the ethoxylated phosphate esters. Thedispersing agent can be one of those commercially referred to as a hyperdispersant.

Suitable dispersing agents for use in the preferred aqueous mediuminclude a polymeric acrylic acid or a salt thereof. Partially or fullyneutralized salts are usable e.g. the alkali metal salts and ammoniumsalts. Examples of dispersing agents are polyacrylic acids, substitutedacrylic acid polymers, acrylic copolymers, sodium and/or ammonium saltsof polyacrylic acids and sodium and/or ammonium salts of acryliccopolymers. Such dispersing agents are typified by polyacrylic aciditself and sodium or ammonium salts thereof as well as copolymers of anacrylic acid with other suitable monomers such as a sulphonic acidderivative such as 2-acrylamido 2-methyl propane sulphonic acid.Comonomers polymerisable with the acrylic or a substituted acrylic acidcan also be one containing a carboxyl grouping. Usually the dispersingagents have a molecular weight of from 1,000 to 10,000 and aresubstantially linear molecules.

In one preferred embodiment of the invention, the dispersing agent inaqueous medium comprises a, preferably water soluble, phosphate or saltof phosphoric acid. Suitable water soluble phosphates include metal,preferably alkali metal, or ammonium phosphates such mono-, di- ortri-sodium phosphate, mono-, di- or tri-potassium phosphate oralternatively a polymeric phosphate, such as a polymeric alkali metalphosphate, for example tri-sodium polyphosphate or sodiumhexametaphosphate. Polymeric phosphates are preferred, particularlysodium hexametaphosphate. In a further preferment, the dispersing agentis the same material which is used to form the phosphate coating layeron the metal oxide particles, especially sodium hexametaphosphate.

In a particular preferment, the dispersing agent in aqueous medium is amixture of one or more of the aforementioned organic dispersing agents,preferably polyacrylic acid, and one or more of the aforementionedphosphorous containing dispersing agents, preferably sodiumhexametaphosphate. The dispersing agent mixture preferably contains thetwo components present at a ratio of 10 to 90%:10 to 90%, morepreferably 30 to 70%:30 to 70%, and especially 40 to 60%:40 to 60% byweight.

Alternatively, the particulate metal oxide may be in the form of alotion or cream of a solid and/or semi-solid dispersion. Suitable solidor semi-solid dispersions may contain, for example, in the range from 50to 90%, preferably 60 to 85% by weight of particulate metal oxideaccording to the present invention, together with any one or more of theliquid media disclosed herein, or a high molecular polymeric material,such as a wax.

The particulate metal oxide and dispersions of the present invention areuseful as ingredients for preparing sunscreen compositions, especiallyin the form of emulsions. The dispersion may further containconventional additives suitable for use in the intended application,such as conventional cosmetic ingredients used in sunscreens. Theparticulate metal oxide according to the present invention may providethe only ultraviolet light attenuators in a sunscreen product accordingto the invention, but other sunscreening agents, such as other metaloxides and/or other organic materials may also be added. For example,the preferred titanium dioxide particles described herein may be used incombination with existing commercially available titanium dioxide and/orzinc oxide sunscreens. Suitable organic sunscreens for use with metaloxide according to the invention include p-methoxy cinnamic acid esters,salicylic acid esters, p-amino benzoic acid esters, non-sulphonatedbenzophenone derivatives, derivatives of dibenzoyl methane and esters of2-cyanoacrylic acid. Specific examples of useful organic sunscreensinclude benzophenone-1, benzophenone-2, benzophenone-3, benzophenone-6,benzophenone-8, benzophenone-12, isopropyl dibenzoyl methane, butylmethoxy dibenzoyl methane, ethyl dihydroxypropyl PABA, glyceryl PABA,octyl dimethyl PABA, octyl methoxycinnamate, homosalate, octylsalicylate, octyl triazone, octocrylene, etocrylene, menthylanthranilate, and 4-methylbenzylidene camphor.

The invention is illustrated by the following non-limiting example.

In this specification the following test methods have been used.

1) Particle Size Measurement of Primary Metal Oxide Particles

A small amount of metal oxide, typically 2 mg, was pressed intoapproximately 2 drops of an oil, for one or two minutes using the tip ofa steel spatula. The resultant suspension was diluted with solvent and acarbon-coated grid suitable for transmission electron microscopy waswetted with the suspension and dried on a hot-plate. Approximately 18cm×21 cm photographs were produced at an appropriate, accuratemagnification. Generally about 300-500 crystals were displayed at about2 diameters spacing. A minimum number of 300 primary particles weresized using a transparent size grid consisting of a row of circles ofgradually increasing diameter, representing spherical crystals. Undereach circle a series of ellipsoid outlines were drawn representingspheroids of equal volume and gradually increasing eccentricity. Thebasic method assumes log normal distribution standard deviations in the1.2-1.6 range (wider crystal size distributions would require many morecrystals to be counted, for example of the order of 1000). The methoddescribed above has been found to be suitable for producing almosttotally dispersed distributions of primary metal oxide particles whilstintroducing minimal crystal fracture. Any residual aggregates (orsecondary particles) are sufficiently well defined that they, and anysmall debris, can be ignored, and effectively only primary particlesincluded in the count.

Mean length, mean width and length/width size distributions of theprimary metal oxide particles can be calculated from the abovemeasurements. Similarly, the median particle volume diameter of theprimary particles can also be calculated.

2) Median Particle Volume Diameter Measurement of Metal Oxide Particlesin Dispersion

A dispersion of metal oxide particles was produced by mixing 105.5 g ofdeionised water, 4.5 g of Calgon (sodium hexametaphosphate, ex FischerScientific), and 90 g of metal oxide. The mixture was passed through ahorizontal bead mill, operating at approximately 1500 r.p.m. andcontaining zirconia beads as grinding media for 15 minutes. Thedispersion of metal oxide particles was diluted with water to between 30and 40 g/l, and the diluted sample was analysed on the Brookhaven BI-XDCparticle sizer in centrifugation mode, and the median particle volumediameter measured.

3) BET Specific Surface Area of Metal Oxide Particles

The single point BET specific surface area was measured using aMicromeritics Flowsorb II 2300.

4) Change in Whiteness and Whiteness Index

A sunscreen formulation was coated on to the surface of a glossy blackcard and drawn down using a No 2 K bar to form a film of 12 μm wetthickness. The film was allowed to dry at room temperature for 10minutes and the whiteness of the coating on the black surface (L_(F))measured using a Minolta CR300 colourimeter. The change in whiteness ΔLwas calculated by subtracting the whiteness of the substrate.(L_(S))from the whiteness of the coating (L_(F)) and expressing the valuerelative to the formulation containing 5% by weight of metal oxideparticles. The whiteness index is the percentage change in whiteness ΔLcompared to a standard titanium dioxide (=100% value) (Tayca MT100T (exTayca Corporation)).

5) Photogreying Index

A metal oxide dispersion was placed inside a 6 cm×3 cm acrylic cell(containing a 2 cm×1.5 cm space), and the cell made air tight byclamping a glass slide over the top, ensuring that no air bubbles werepresent. The initial whiteness (L_(I)) was measured using a MinoltaCR300 colourimeter. The cell was then placed on a turntable revolving at30 rpm and exposed to UV light for 2 hours (a UV lamp containing 4TL29D,16/09 tubes mounted 12 cm from the cell), and the whiteness(L_(T)) remeasured. The photogreying index ΔL=L_(I)−L_(T).

6) Sun Protection Factor

The Sun Protection Factor (SPF) of a sunscreen formulation wasdetermined using the in vitro method of Diffey and Robson, J. Soc.Cosmet. Chem. Vol. 40, pp 127-133,1989.

EXAMPLE 1

2 moles of titanium oxydichloride in acidic solution were reacted with 6moles of NaOH in aqueous solution, with stirring, in a 3 litre glassvessel. After the initial reaction phase, the temperature was increasedto above 70° C., by heating at a rate of approximately 1° C./min, andstirring continued for at least another 60 minutes. The mixture was thenneutralised by the addition of NaOH in aqueous solution, and allowed tocool below 70° C.

To the resultant dispersion, a solution of sodium hexametaphosphate wasadded, equivalent to 4% by weight P₂O₅ on TiO₂ weight over a period of20 minutes. The mixture was stirred for at least another 30 minutes. Anacidic solution of aluminium sulphate was added, equivalent to 3% byweight Al₂O₃ on weight of TiO₂ was added over a period of 20 minutes,and the reaction mixture again stirred for at least a further 30minutes. An alkaline solution of sodium aluminate was added over aperiod of 40 minutes, equivalent to 6% by weight Al₂O₃ on TiO₂ weight,maintaining pH between 10.0 and 10.5 by addition of 36% hydrochloricacid as required. The reaction mixture was stirred for a further 45minutes. The temperature of the reaction mixture was maintained at 50°C. throughout the reactions.

The dispersion was neutralised to pH 5.5 to 6.0 by adding 36%hydrochloric acid solution over 30 minutes. The neutralised slurry wasaged for 15 minutes whilst being stirred. The slurry was then filteredto produce a filter cake, which was then washed repeatedly withdemineralised water until the cake conductivity (when a small sample wasreslurried to 100 μl) was less than 500 μs. The filter cake was dried inan oven at 105° C. for 16 hours and then micropulverised using a hammermill to produce particulate titanium dioxide having a phosphate coating.

A dispersion of metal oxide particles was produced by mixing 105.5 g ofdeionised water, 4.5 g of Calgon (sodium hexametaphosphate, ex FischerScientific), and 90 g of the titanium dioxide produced above. Themixture was passed through a horizontal bead mill containing zirconiabeads as grinding media, operating at approximately 1500 r.p.m. for 15minutes. A fluid dispersion was produced.

The particulate titanium dioxide and dispersion were subjected to thetest procedures described herein, and exhibited the followingproperties:

Primary Particle Size

-   i) Mean length 64 nm,-   ii) Mean width=14.5 nm,-   iii) Mean aspect ratio=4.4, and-   iv) D (v,0.5)=26.5 nm.    Dispersion Particle Size-   i) D (v,0.5)=23.5 nm,-   ii) 16% by volume of particles have volume diameter less than 18 nm,-   iii) 84% by volume of particles have volume diameter less than 34    nm, and-   iv) BET specific surface area=100 m²/g.

0.1 g of the milled titanium dioxide dispersion produced above wasdiluted with 100 ml of water, and then further diluted with water in theratio sample:water of 1:19. The total dilution was 1:20,000.

The diluted sample was then placed in a spectrophotometer (Perkin-ElmerLambda 2 UVNIS Spectrophotometer) with a 1 cm path length and theabsorbance, of UV and visible light measured. Extinction coefficientswere calculated from the equation A=E.c.I, where A=absorbance,E=extinction coefficient in litres per gram per cm, c=concentration ingrams per litre, and I=path length in cm.

The results were as follows; E₅₂₄ E₃₀₈ E₃₆₀ E(max) λ(max) 0.9 46.5 6.5968 276

The titanium dioxide dispersion produced above was used to prepare asunscreen formulation having the following composition; % by weightPhase A: ARLACEL 165 (trade mark, ex Uniqema) 3.0 Lorol C18 (ex Cognis)0.5 SPAN 60 (trade mark, ex Uniqema) 3.0 TWEEN 60 (trade mark, exUnqema) 0.4 Petroleum jelly 3.0 DC 200 fluid (ex Dow Corning) 1.0 ESTOL3609 (trade mark, ex Uniqema) 6.0 PRIPURE 3759 (trade mark, ex Uniqema)8.0 Antaron V-220 (ex ISP) 2.5 Phase B: Water; pure 44.7 Glycerine BP5.0 Aloe Vera Gel 10:1 0.7 Keltrol RD (ex Nutrasweet Kelco) 0.1 Titaniumdioxide dispersion 18.8 Phase C: Water; pure 2.5 Phenonip (ex Clariant)0.6 Germall 115 (ex ISP) 0.3

The ingredients of aqueous phase B were mixed and heated to 75° C. Theingredients of phase A were mixed and heated to 70-75° C. and slowlyadded to the phase B with high shear mixing, followed by homogenizationin a Ultra Turrax T25 homogeniser. The mixture was cooled with moderatestirring and the preservative phase C added at 45° C. Stirring wascontinued until the mixture cooled to room temperature. The SunProtection Factor of the sunscreen formulation was measured and a valueof 20 was obtained.

The above example illustrates the improved properties of a particulatemetal oxide, dispersion and sunscreen product according to the presentinvention.

1. A particulate metal oxide having a coating layer comprising phosphatewherein the mean length of the primary particles is in the range from 50to 90 nm, and the mean width of the primary particles is in the rangefrom 5 to 20 nm.
 2. A dispersion comprising particles of metal oxidehaving a coating layer comprising phosphate in a dispersing mediumwherein the mean length of the primary particles is in the range from 50to 90 nm, and the mean width of the primary particles is in the rangefrom 5 to 20 nm.
 3. A metal oxide or dispersion according to claim 1,wherein the primary particles have a mean length in the range from 55 to73 nm, preferably 60 to 70 nm, and a mean width in the range from 10 to18 nm, preferably 12 to 17 nm.
 4. A metal oxide or dispersion accordingto claim 1, wherein at least 70% of the particles have a length in therange from 55 to 73 nm.
 5. A metal oxide or dispersion according toclaim 4 wherein at least 70% of the particles have a length in the rangefrom 60 to 70 nm.
 6. A metal oxide or dispersion according to claim 1,wherein the median particle volume diameter of the primary particles isin the range from 25 to 31 nm.
 7. A metal oxide or dispersion accordingto claim 1, wherein the median particle volume diameter of the particlesin dispersion is less than 45 nm, preferably in the range from 20 to 30nm.
 8. A metal oxide or dispersion according to claim 1, wherein no morethan 16% by volume of the particles in dispersion have a volume diameterof less than 18 nm, preferably less than 20 nm.
 9. A metal oxide ordispersion according to claim 1 wherein more than 84% by volume of theparticles in dispersion have a volume diameter of less than 34 nm,preferably less than 30 nm.
 10. A metal oxide or dispersion according toclaim 1, wherein the amount of phosphate in the coating layer is in therange from 0.1 to 12% by weight of phosphorous, based on the weight ofmetal oxide core particles.
 11. A metal oxide or dispersion according toclaim 1, wherein the amount of metal cation in the coating layer is inthe range from 0.2 to 20% by weight, based on the weight of metal oxidecore particles.
 12. A metal oxide or dispersion according to claim 11wherein the metal comprises aluminium.
 13. A metal oxide or dispersionaccording to claim 1, wherein the coating layer additionally comprisesaluminium oxide and/or aluminium hydroxide.
 14. A metal oxide ordispersion according to claim 13 wherein the coating layer comprisesaluminium phosphate and (aluminium oxide and/or aluminium hydroxide) ata ratio by weight in the range from 0.1 to 1.8:1
 15. A dispersionaccording to claim 2 comprising an aqueous medium.
 16. An aqueousdispersion according to claim 15 comprising at least 30% by weight ofmetal oxide particles.
 17. An aqueous dispersion according to claim 15comprising a phosphorous containing dispersing agent.
 18. A metal oxideor dispersion according to claim 1, wherein the particles of metal oxidehave an extinction coefficient at 524 nm (ES24) in the range from 0.2 to1.5, preferably 0.4 to 1.0 I/G/CM.
 19. A metal oxide or dispersionaccording to claim 1, wherein the particles of metal oxide have anextinction coefficient at 360 nm (E360) IN the range from 5 to 10,preferably 5.5 to 8 I/G/CM.
 20. A particulate metal oxide having acoating layer comprising phosphate, and having an extinction coefficientat 524 nm (ES24) in the range from 0.2 to 1.5 I/G/CM, an extinctioncoefficient at 450 nm (E4SO) in the range from 0.1 to 2.0 I/G/CM, anextinction coefficient at 360 nm (EGO) in the range from 5 to 10 I/G/CM,an extinction coefficient at 308 nm (E308) in the range from 35 to 65I/G/CM, a maximum extinction coefficient E (max) in the range from 50 to80 I/G/CM, and a K (max) in the range from 260 to 290 nm.
 21. A metaloxide according to claim 20 having a photogreying index in the rangefrom 1 to
 10. 22. A sunscreen product comprising a metal oxide ordispersion as defined in claim
 1. 23. A sunscreen product according toclaim 22 which is transparent when applied to the skin and has a changein whiteness AL in the range from 0.5 to 2.5.
 24. A sunscreen productaccording to claim 22 having a whiteness index in the range from 10% to80%.
 25. A sunscreen product according to claim 22 having a SunProtection Factor (SPF) of greater than 15, preferably greater than 20.26. The use of a metal oxide or dispersion as defined in claim 1 in themanufacture of a sunscreen having reduced whiteness.